Effects of estrogen in vivo on coronary vascular resistance in perfused rabbit hearts

Estrogen or its vehicle was given daily to three groups of ovariectomized rabbits for various lengths of time, after which coronary flow was measured in their isolated perfused hearts. In one group, intramuscular injections of estrogen (40 micrograms/kg) for 7 or 14 days increased coronary flow by 40-50% (P < 0.05). In rabbits given estrogen intramuscularly for 7 days, the coronary flow returned to the basal level within 7 days after the estrogen injections were discontinued. In a second group of animals, intravenous injections of estrogen (10 micrograms/kg) for 4 days increased the coronary flow by 45% (P < 0.01). In a third group, we administered the estrogen transdermally for 4 days, and we measured the plasma estrogen levels at the end of this period. The coronary flow in this group was increased by 52% (P < 0.001), and the plasma estrogen levels ranged from 39 to 800 pg/ml. In all groups of experiments, the increments in coronary flow evoked by estrogen were virtually abolished by NG-nitro-L-arginine, an inhibitor of nitric oxide synthase. We conclude that estrogen regulates coronary blood flow, in part by upregulating nitric oxide synthase in the coronary vasculature.